1. Field of the Invention
This invention relates to a reversible thermosensitive coloring composition capable of developing and decolorizing a colored image repeatedly by utilizing a coloring reaction between an electron-donor coloring compound and an electron-acceptor compound. This invention also relates to a reversible thermosensitive coloring recording medium, a recording and display method, a display medium, and an image display apparatus using the reversible thermosensitive coloring recording medium.
2. Discussion of the Background
Conventionally, thermosensitive recording media utilizing a coloring reaction between electron donor-coloring compounds (hereinafter, referred to as coloring agents) and electron-acceptor compounds (hereinafter, referred to as color developers) are widely known and have been employed in various fields, for instance, for use with terminal printers for computers, facsimile apparatus, automatic ticket vending apparatus, printers for scientific measuring instruments, and printers for CRT medical measuring instruments. However, such conventional thermosensitive recording media for use with the above-mentioned products do not have reversibility with respect to the coloring or decolorizing in image formation, so that the color development and the decolorization cannot be alternately performed repeatedly.
Among published patents, there are several proposals for thermosensitive recording media which can reversibly develop and decolorize or erase colored images utilizing a coloring reaction between coloring agents and color developers. For example, a thermosensitive recording medium using the combination of phloroglucinol and gallic acid as color developers is disclosed in Japanese Laid-Open Patent Application 60-193691. The images obtained by developing a color using gallic acid and phloroglucinol upon the application of heat thereto, is erased when coming into contact with water or aqueous vapor. In the case where such types of thermosensitive recording media are employed, there are difficulties in imparting water-resisting properties to the recording medium and obtaining stable recording preservability. Furthermore, there is another problem in that a large image erasing apparatus is required to erase the displayed image on the above-mentioned recording medium.
In Japanese Laid-Open Patent Application 61-237684, a rewritable optical information recording medium which employs compounds such as phenolphthalein, thymolphthalein and bisphenol as color developers is disclosed. In the above optical information recording medium, colored images are formed by applying heat thereto and gradually decreasing the temperature thereof. The colored images can be decolorized or erased by applying heat to the recording medium at a higher temperature than the image developing temperature, and then rapidly cooling the recording medium. In the case of this optical information recording medium, the color developing and decolorizing steps are complicated and the contrast of the colored image is not satisfactory with some color remaining on the erased image which is obtained by erasing the displayed image.
In Japanese Laid-Open Patent Applications 62-140881, 62-138568, and 62-138556, thermosensitive recording media using a homogeneously dissolved composition of a coloring agent, a color developer and a carboxylic acid ester are disclosed. The above recording media can assume a completely colored state at a low temperature, a completely decolorized state at a high temperature, and can maintain the colored state or the decolorized state at a temperature midway between the above-mentioned low temperature and high temperature. When heat is applied to the recording media using a thermal head, a white image (decolorized image), which is similar to a photographic negative, is recorded on the colored background. Accordingly, the usage of above recording media is limited. It is also necessary that the temperature of the recording media be maintained within a specific range in order to preserve the recorded image on the recording media.
In Japanese Laid-Open Patent Applications 2-188294 and 2-188293, there are disclosed a thermosensitive recording medium utilizing a salt of gallic acid and a higher aliphatic amine, and a thermosensitive recording medium utilizing a salt of a bis(hydroxyphenyl)acetic acid or burytic acid and a higher aliphatic amine. These salts have a reversible color developing and decolorizing function. With this type of recording medium, a colored image can be developed in a specific temperature range with the application of heat thereto, and can be decolorized or erased by applying heat thereto at a higher temperature than the above-mentioned specific temperature range. However, since the color developing effect and the decolorizing effect competitively occur, it is difficult to thermally control these effects by changing the temperature of the recording medium. Therefore, it is difficult to obtain a stable image contrast.
As mentioned above, the conventional reversible thermosensitive recording media utilizing the coloring reaction between a coloring agent and a color developer have many problems and are unsatisfactory for use in practice. In particular, a multiple colored image on a conventional reversible thermosensitive recording medium is completely unsatisfactory.
The inventors of the present invention have previously disclosed a thermosensitive recording medium comprising as the main components a specific fluoran compound and an ascorbic acid-6-o-acyl derivative in Japanese Laid-Open Patent Application 63-173684. This recording medium can assume a color development state with the application of heat thereto at a high temperature of 90.degree. C. or more, and can assume a decolorized state with the application of heat thereto again at temperatures in the range of 65.degree. to 90.degree. C. The recording medium has the characteristics that the image recording and erasing can be performed only by the application of heat.
However, the color development state of the above-mentioned thermosensitive recording medium is not always stable. For instance, when water comes into contact with the surface of the thermosensitive recording medium in the color development state, the colored image is decolorized and erased, and when the thermosensitive recording medium with the colored image printed thereon is stored under high humidity, decolorization occurs and the image density is decreased. Even when heat is again applied to the recording medium to erase the image, the decolorization is not satisfactory. In other words, the density of the image is not decreased to the level of that of the background and the image can still be observed after decolorization. Therefore, these problems must be solved in order to use this type of thermosensitive recording medium in practice.